Figure 5.

Investigations of β-sheet structures in prion fibrils, through measurements of intermolecular ¹³C-¹³C magnetic dipole couplings with the PITHIRDS-CT technique.²⁰ (a) Ideal in-register parallel and antiparallel β-sheets. (b) Ideal PITHIRDS-CT curves for an in-register parallel β-sheet with a single carbonyl ¹³C label at any residue in each peptide chain (cyan) and for antiparallel β-sheets with a single carbonyl ¹³C label at either of two residues (orange and green). (c) PITHIRDS-CT data for Sup35NM fibrils, prepared with ¹³C labels at backbone carbonyl sites of all Tyr (circles) or Leu (triangles) residues. Data were obtained from lyophilized (blue) or fully hydrated (red) samples.⁴¹ All 20 Tyr residues are in the N domain of Sup35NM, while 7 of 8 Leu residues are in the M domain. (d) PITHIRDS-CT data for wild-type Ure2p_1–89 fibrils (triangles), prepared with ¹³C labels at backbone carbonyl sites of all Leu residues, and for two “shuffled prion” fibrils⁴⁰ with Leu at the indicated residue numbers (squares and circles). (e) PITHIRDS-CT data for HET-s fibrils, residues 218–289, prepared with ¹³C labels at backbone carbonyl sites of the indicated residues, in separate samples. (f) Side and cross-sectional views of the HET-s fibril model developed by van Melckebeke et al. (PDB 2KJ3).⁴ Residues 224–248 and 259–289 are shown, corresponding to the most highly ordered and rigid segments. Labeled carbonyl sites are represented by large spheres, colored according to panel e. In the β-helical core structure of HET-s fibrils, only Leu carbonyls have ~5 Å nearest-neighbor distances. Signal contributions from labeled sites outside the core are uncertain. (All PITHIRDS-CT data are corrected for natural-abundance ¹³C contributions.⁴¹)